#include <iostream>
#include <vector>
using namespace std;

class Solution {
public:
    int takeAttendance(vector<int>& records)
    {
        int n = records.size();
        int sum1 = (n + 1) * (0 + n) / 2;
        int sum2 = 0;
        for (int i = 0; i < n; i++)
        {
            sum2 += records[i];
        }
        return sum1 - sum2;
    }
};

int main()
{
    Solution s;
    vector<int> v = { 0,1,2,3,5 };
    cout << s.takeAttendance(v) << endl;
    return 0;
}